Conventionally, there has been known an optical apparatus performing focus detection of an imaging optics by a pupil division method (see, for example, Patent Document 1).
In the above-described apparatus, an electro-optic element composed of the combination of a grating and liquid crystal is inserted on a pupil plane of the imaging optics, and lights that have passed through different regions in an exit pupil are polarized in different directions. Then, beams of the lights are imaged on different regions of an imaging element capturing images of the imaging optics, and phase difference AF is performed based on image data thereof.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-106435